Recently, many studies have been performed on mechanisms and therapeutic agents for diseases caused by various immune disorders or immune depression caused by adverse reactions of carcinostatic drugs. It is known that thymulin, a nonapeptide produced in the thymus, forms a complex with zinc and improves a depressed immunity. It is suggested that it could be an effective drug on immunodeficiency and autoimmune diseases (Med. Oncol. & Tumor Pharmacother. 6, 25-29, 1989).
However, there remain many problems in the practical use of thymulin. For example, the availability of thymulin is limited due to a small yield in the thymus and duration of the activity thereof is short because thymulin is easily decomposed by endogenous enzymes.
Therefore, synthetic compounds are desired which have a long lasting activity and can be prepared in a large amount. But few studies have been performed on such synthetic compounds having thymulin-like activities.
The inventors focused on the action mechanisms of thymulin forming a complex with zinc and studied to synthesize various novel compounds having sulfur atoms in the side chains.
The inventors also examined the effect of the obtained novel compounds. As the result of the precise studies, the inventors found that lactone or lactam compounds having sulfur atoms in the side chains showed excellent thymulin-like activities. Although many studies have been performed on lactone or lactam compounds, few compounds having sulfur atoms in the side chains were known. As mentioned later, the structural characteristic of the compounds of this invention of the formula [I] is that two intramolecular sulfur atoms exist in the side chains of lactone or lactam compounds. Such lactone or lactam compounds have not been reported.